Pic does not work properly

[Armando Herjim]

Hello. My problem is that when I power up my pic it seems to stop right after the first instruction (which is to set a bit, in this case it is a led), but I just found out that if I put my finguer on the pin that connects to ground it works fine!! Does anybody know why is this happening? I can´t be puting my finger on it all the time!!
Thank you.

[Pic_User]

Hi Armando, you seem to have put you FINGER on the problem.
At first I thought "Of course, because this is a DIGITAL circuit".

My own "HANDS ON experience" has shown this type of mysterious phenomenon to happen when the MCLR pull-up resistor is forgotten.

Maybe, missing or wrong value capacitors on the crystal.

-Adam-

[Armando Herjim]

Hi Adam. Look, I have a pull-up resistor to MCLR just like the specification sheet sugests, I´m unsing a 10 Mhz Crystal and 33 pF capacitor. In fact I am changing the value of the crystal from 4 Mhz to 12 Mhz just that I also change te capacitors from 33 pF to 15pF because I´m trying to get a baudrate of 115200, so that´s the why I´m experimenting with diferent crystals. And one more thing, I also found that when I connect the serial cable it always works, I figure that´s because the external part of the serial cable is connected to gnd... what do you think?

Thanks a lot!

[Pic_User]

Hi Armando,

I see you have done everything exactly by the book.
It sound like you are correct about the serial cable adding a ground. Same with your finger on the LED.

What do you use for a power supply? Low noise? Is the ground connection between the supply and your PIC circuit good?

Do you have two capacitors near the power pins of the PIC to ground.
Something like a 10 microfarad. and a 0.1 microfarad, in parallel.

I would double check all the intended ground connections to see if some might be forgotten.

Maybe some of the unused / un-terminated pins are picking up noise.

Any other suggestions for Armando out there?

-Adam-

[Armando Herjim]

Hey Adam, thanks for your help. I appreciated too much. Listen, you wrote this:

"I would double check all the intended ground connections to see if some might be forgotten.

Maybe some of the unused / un-terminated pins are picking up noise."

Are you suggesting that I should connect all the unused pins to ground?? And what is exactly all the intended ground connections? I mean, I only have de gnd pin connected to ground, well and the capacitor too. I also have the 10 microfarad capacitor in parallel with the 100 nanofarad capacitor...

Thanks for your help!

Armando.

[Armando Herjim]

Hey everybody. Hey adam I got it, or at least I think so. At first I though that the pic wasn´t working properly for some strange reason and that when I had the pic on the board it would work just fine, but it didn´t. But after a while I found that the problem was on pin 38, wich is the RB5/KBI1/PGM. When I put this pin to ground it work perfectly fine!! Even on the protoboard. I don´t know what´s the reason of this, so if anyone could explain me I´d be very thankfull. Thank you.

Armando.

[mister_e]

what is your PIC?
Did you disable the low-Voltage Programming fuse?
What about your schematic?

[Armando Herjim]

Hello Steve. My pic is 18F4550. I don´t know what is the low-voltage fuse, I´ll check it... and the schematic... well it´s just the pic with its power pins the reset circuit and the oscillator with its respective capacitors and I have a led on portb.7, portb.6 and portb.5. I´m also using the USART pins to communicate a CMUcam to my pic.... and that´s it!...... sorry I don´t post the schematics but right now I don´t have any but I´ll make one. Thanks!!

Armando

[mister_e]

mmm, looking at the default fuse setting, the low-voltage programming is already disabled, and the value pass great to MPLAB, so just double check the configuration fuse setting before you program your PIC, and be sure that the low-voltage programming is set to OFF or Disabled.

After that... i don't see any other software reason why it shouldn't work...maybe i need to sleep too.. who knows :)

Just post your code and schematic and i'll test it here.